I have not flown an ILS since 2008 in actual conditions. I regularly practice an ILS, but if ATC is using an ILS, I will request the RNAV. I have never been turned down, although vectors to final is the normal method of joining the approach because the path will overlay the ILS and I won't interfere with others flying the ILS. All that said, I would keep my ILS/VOR system as a backup. In the older installations with a separate GPS and Nav/Com, the primary CDI is usually switched between GPS and Nav and in most cases, the relay has spare poles to add the GS signals. If not, update the relay and keep the SL30, after all you still need a Com unit.

I'd be a lot more concerned about the actual interference testing by the U.S. military than a hypothetical attack by the Russians.

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I see these notices all the time and I have yet to lose a signal. I think a lot of the arrival/departure procedures into and out of the LA Basin are RNAV...do they quit using those during the testing period?

I have not flown an ILS since 2008 in actual conditions. I regularly practice an ILS, but if ATC is using an ILS, I will request the RNAV. I have never been turned down, although vectors to final is the normal method of joining the approach because the path will overlay the ILS and I won't interfere with others flying the ILS. All that said, I would keep my ILS/VOR system as a backup. In the older installations with a separate GPS and Nav/Com, the primary CDI is usually switched between GPS and Nav and in most cases, the relay has spare poles to add the GS signals. If not, update the relay and keep the SL30, after all you still need a Com unit.

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The fly in that ointment is the 175/375 isn't approved in the AFMS as a switched source, and it doesn't have the required annunciation pinouts. But I think I've decided to go with the 175 to drive my 525A and stec 60-2 for lpv appchs. Keep one sl30 as a separate loc/ils capability, and the second SL30 as a nav/com without a cdi, so if tshtf I can cross check radials using the integrated display and be on my merry way /A.
Thanks for the input, John.

The AFMS does not limit if the CDI source is switchable from GPS or VLOC. So there is no reason not to have one of your SL30 as a VLOC source and a switch/relay/Source annunciator to switch between GPS and VLOC.

The external CDI is mentioned in a few places in the Limitations section of the AFMS, see this partial quote (reformatted):

2.2 Minimum Equipment
The GPS 175/GNX 375 must have the following system interfaces fully functional in order to be used for primary navigation during IFR operations:
External HSI/CDI/EHSI:, Number installed: 1 or more: Required for IFR: 1
External APPR and LOI Annunciator; Number Installed: See Note 1; Number required for IFR: 1

Note 1: Certain installations require an external APPR and LOI annunciator light. If installed, these
annunciators must be fully functional to use the GPS 175/GNX 375 GPS navigation for IFR
operations

2.6 System Use
The only approved sources of course guidance are on the external CDI, HSI, or EHSI display.
The moving map and CDI depiction on the GPS 175/GNX 375 display are for situational awareness only and are not approved for course guidance.

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Although not a limitation (which is a legal requirement), section 4 of the AFMS for Normal Procedures deals with the topic of HSI/EHSI Operation. The caution note make no sense unless the installation supported a capability to select the source of GPS or VLOC on the same HSI.

4.3 HSI and EHSI Operation
If an HSI is used to display navigation data from the GPS 175/GNX 375 the pilot should rotate the course pointer as prompted on the GPS 175/GNX 375. If an EHSI is used to display navigation data from the GPS 175/GNX 375 the course pointer may autoslew to the correct course when using GPS navigation. For detailed information about the functionality of the EHSI system, refer to the
FAA approved Flight Manual or Flight Manual Supplement for that system.

CAUTION: The pilot must verify the active course and waypoint for each flight plan leg. The pilot must verify proper course selection each time the CDI source is changed from GPS to VLOC.

The AFMS does not limit if the CDI source is switchable from GPS or VLOC. So there is no reason not to have one of your SL30 as a VLOC source and a switch/relay/Source annunciator to switch between GPS and VLOC.

The external CDI is mentioned in a few places in the Limitations section of the AFMS, see this partial quote (reformatted):

Although not a limitation (which is a legal requirement), section 4 of the AFMS for Normal Procedures deals with the topic of HSI/EHSI Operation. The caution note make no sense unless the installation supported a capability to select the source of GPS or VLOC on the same HSI.

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Then I'm a little confused as to why the source switching wasn't spec'd in the external switching section, as it was in the GNS 4xx/5xx manual or the GTN 6xx/7xx manual, nor are there any interconnect diagrams as there are in the GX series IM's. The only source switching shown in the IM is for the Ki208A and 209A.

As a practical matter, 12v remote solenoids are difficult to find and very expensive, and add significantly to the complexity of the install. I'm not sure the juice is worth the squeeze.

Unless you were based at VNY, MCC or any other of the very rare airports that don't have a decent (or any) GPS approach, you are getting much more if you choose LPV over ILS. Not only are you getting LPV, but all the non-LPV GPS approaches. Those are generally superior to some crappy VOR or even LOC only approach.

I prefer to use an LPV approach over an ILS approach. The LPV approaches tend to be more stable and less susceptible to issues like prop shadowing (the 310 was very bad about this). Plus LPVs are more available at a larger number of airports than ILSs are. Pretty much all airports with an ILS also have an LPV to equal minimums, but many airports with LPVs do NOT have an ILS.

That said, I would not want to give up ILS for the simple reason that at busier airports you will often be assigned an ILS, and in some cases if you ask for the LPV you will be told "No." So then that puts you in the position of having to say "Unable" and wondering what NY approach will do with you, or else shooting the LPV approach to the same one (assuming there is one) and hoping you don't get in trouble for it. I don't like that position.

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On the first point, I totally agree. GPS approaches simply don't have the radio sensitivity issues that ILS approaches do, and light aircraft with GPSS autopilots fly them much better when you need to shoot with a little help than they do an ILS. Your information from the CDI is always going to be correct, no matter how close you get to the radio.

On the second, when you have an LPV overlaid on an ILS, there is zero reason for a controller to deny. They use the same waypoints and even FAFs. More and more, I'll even have controllers clearing "ILS or RNAV." If a controller is denying in that situation, they are just being jerks.

Having just retaken the IR written, if you file for an LPV approach, and you require an alternate, the alternate (assuming less than VMC from MEA to runway) must be a different approach type, (ILS or LOC) correct? That alone would be enough reason to me.

I wonder if that includes the cost of satellite maintenance, Air Force staff required to monitor the system, and other required GPS costs. I think you'd have to for an apples to apples.

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Not from an FAA perspective. The GPS constellation is going to exist for the military regardless of other users. Might as well take advantage of it if you're the FAA. No more electronic stuff to maintain, just flight test it every so often and you're good.

I am preparing to be disappointed in the Garmin 175. Looking over the IM, it looks like it will support my HSI and A/P, but won't share it with my SL30. There are a couple dedicated pins missing on the J connector to support the ACU, like OBS HOLD/Susp, term, wpt. The only two outputs are LOI and APPCH. No interconnect diagram either. Not a good sign, I think.

So that leaves the question. If the 175 won't support sharing an HSI, would it be worth the trade off, LPV for ILS?

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I'm just getting back into flying again, so not totally up to speed, so I might be way off. But I am looking to buy an airplane that is equipped with two NavComs but lacks a GPS and ADSB.
So it occured to me that a GNX 375 would be great. But then I realized it wouldn't do ILS. But the ILS approaches are in the database, just come with a warning that it's for monitoring only. So it occurred to me that you could allow the GPS to provide guidance to the AP but you would have to have a NavCom tuned to the ILS and displayed on an indicator. Then you could let the AP fly the LPV provided you monitor the ILS indicator to be sure you are following LOC and GS.

Am I crazy, misinformed, or would that work and be legal. It seems to me that as long as the ILS indications are in the right place, how you achieve that is up to you.

I'm just getting back into flying again, so not totally up to speed, so I might be way off. But I am looking to buy an airplane that is equipped with two NavComs but lacks a GPS and ADSB.
So it occured to me that a GNX 375 would be great. But then I realized it wouldn't do ILS. But the ILS approaches are in the database, just come with a warning that it's for monitoring only. So it occurred to me that you could allow the GPS to provide guidance to the AP but you would have to have a NavCom tuned to the ILS and displayed on an indicator. Then you could let the AP fly the LPV provided you monitor the ILS indicator to be sure you are following LOC and GS.

Am I crazy, misinformed, or would that work and be legal. It seems to me that as long as the ILS indications are in the right place, how you achieve that is up to you.